EMU impulse antenna with controlled directionality and improved impedance matching

ABSTRACT

An electromagnetic energy source for emitting pulses of electromagnetic energy includes a sonde assembly and an energy storage capacitor. The energy storage capacitor has an electrode mounted in the sonde assembly and operable to generate an electric field, and a capacitive charge storage medium surrounding the electrode. A communication cable extends through a tubular member to the electrode. A fast-closing switch is positioned such that when the fast-closing switch is in a closed position, a circuit is formed that discharges the electrode.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority to and thebenefit of, co-pending U.S. application Ser. No. 15/878,123, filed Jan.23, 2018, titled “EMU Impulse Antenna With Controlled Directionality andImproved Impedance Matching,” which is a continuation in part of, andclaims priority to and the benefit of, co-pending U.S. application Ser.No. 15/458,772, filed Mar. 14, 2017, titled “EMU Impulse Antenna,” thefull disclosure of each which is incorporated in this disclosure byreference in its entirety for all purposes.

BACKGROUND 1. Field of the Disclosure

The present disclosure relates to imaging sub-surface structures,particularly hydrocarbon reservoirs and fluids within the hydrocarbonreservoirs, and more particularly to electromagnetic energy sources forelectromagnetic surveying of sub-surface structures.

2. Description of the Related Art

Some electromagnetic (EM) surveying systems used in geophysics provideelectromagnetic energy for traveling through a subsurface hydrocarbonreservoir for electromagnetic imaging of the subsurface hydrocarbonreservoir. Multiple sources and receivers can be positioned either in abore that extends to the subsurface hydrocarbon reservoir or an earthsurface over the subsurface hydrocarbon reservoir. In this way, thedirection, velocity and saturation of injected fluids (such as duringwater flood) can be monitored. The system can also be used to locateby-passed oil and detect conductivity zones (such as fracture corridorsand super-k zones) to provide early warning of water break-through. Suchoperations can assist in optimizing reservoir management and preventingoil bypass for improving volumetric sweep efficiency and productionrates.

Some current EM systems cannot easily match impedance of the EM systemto the geological matrix of the subsurface hydrocarbon reservoir forincreasing transmission efficiency. Some current EM systems use a cableto provide power to the EM transmitter. However, these systems have beenshown to have difficulty transferring a crisp pulse from the powersupply down a cable, and then matching that pulse into the antenna. Inaddition, the cabling can also transmit a signal, which makes theresulting measurements unclear.

In some current systems, the system can be deployed by wireline undergravity tension in a vertical borehole or along a semi-rigid means, suchas coiled tubing, if used in a horizontal or lateral borehole. Suchdeployment systems can introduce some asymmetry into the radiationpattern from the proximal half of the antenna, which could negativelyimpact data quality and interpretation of gathered data.

SUMMARY OF THE DISCLOSURE

Embodiments of this disclosure provide systems and methods for impedancematching to the formation and providing a symmetrical radiation pattern.Systems and methods of this disclosure are particularly suited fordeployment in long term sub-surface applications.

Embodiments of this disclosure combine a slow-wave antenna with energystorage and pulse forming elements to form a high power, small aperturetransmitting antenna that is suited for downhole electromagneticinterrogation technologies, such as for electromagnetic imaging of asubsurface hydrocarbon reservoir. Systems and methods described in thisdisclosure provide a transmitter that is compact, has an instantaneoushigh power output and generates a clean signal that is free of materialringing or distortion, such as a non-attenuated signal with a highsignal to noise ratio. As used in this disclosure, a “high power” isconsidered to be a power in a range of a number of kilowatts to a numberof megawatts.

Systems and methods of this disclosure eliminate external transmissionlines to a switched power supply and radiate an electromagnetic pulsewith a wavelength larger than the physical length of the antennastructure. The antenna elements of embodiments of this disclosure areused as capacitive energy storage elements. A fast-closing (normallyopen) switch, such as a triggered spark gap, is provided to initiatepulsed transmission. The magnitude and shape of the current pulse willdepend on the voltage, capacitance, inductance and the resistance of theantenna elements and the resistance of the switch. As used in thisdisclosure, a “massive current” is considered to be in a current in therange of 100-1000 amperes (A) or more. Systems and methods of thisdisclosure therefore combine energy storage, pulse formation andradiating elements into a single structure, eliminating the need forimpedance matching between separate distributed components for theserespective functions.

Systems and methods of this disclosure further eliminate the problem ofload matching between a power supply, cable or transmission-line, andantenna. With the energy storage element and switch inside thetransmitting antenna element, the cable between the two is eliminated,minimizing reflections and losses in the system. As used in thisdisclosure, an “EMU” antenna is an acronym for an antenna having anelectric permittivity (E), and a magnetic permeability (MU).

In an embodiment of this disclosure, an electromagnetic energy sourcefor emitting pulses of electromagnetic energy includes a sonde assemblyand an energy storage capacitor. The energy storage capacitor has anelectrode mounted in the sonde assembly that is operable to generate anelectric field, a capacitive charge storage medium surrounding theelectrode, and a communication cable extending through a tubular memberto the electrode. A fast-closing switch is positioned such that when thefast-closing switch is in a closed position, a circuit is formed thatdischarges the electrode.

In alternate embodiments of this disclosure, a high voltage power supplycan be in communication with the electrode and in communication with thecommunication cable. The sonde assembly can include a downhole sectionaxially aligned with, and spaced from, an uphole section, each of thedownhole section and the uphole section having an energy storagecapacitor. The communication cable can extend through the uphole sectionof the sonde assembly. Alternately, the tubular member can act as aground.

In other alternate embodiments, the communication cable can be incommunication with a high voltage power supply and can transmit acontrol signal. The capacitive charge storage medium can be formed of amaterial that includes iron particles and an epoxy matrix. Theelectromagnetic energy source can further include a plurality ofelectromagnetic energy sources emitting pulses of electromagnetic energyto travel through a subsurface hydrocarbon reservoir. The capacitivecharge storage medium can have a magnetic permeability and electricpermittivity selected to result in an impedance of the electromagneticenergy source that corresponds to an impedance of the subsurfacehydrocarbon reservoir. The electromagnetic energy source can be movableto a succession of locations in a well borehole for emitting the pulsesof electromagnetic energy at the succession of locations for travelthrough a subsurface hydrocarbon reservoir.

In an alternate embodiment of this disclosure, an electromagnetic energysource for emitting pulses of electromagnetic energy includes a sondeassembly having a downhole section axially aligned with, and spacedfrom, an uphole section. An energy storage capacitor includes anelectrode mounted in each of the downhole section and the uphole sectionof the sonde assembly that is operable to generate an electric field,and a capacitive charge storage medium mounted in each of the downholesection and the uphole section of the sonde assembly and surroundingeach electrode. A fast-closing switch is located between the electrodeof the downhole section and the electrode of the uphole section of thesonde assembly. A communication cable extends through the uphole sectionof the sonde assembly.

In another alternate embodiment of this disclosure, an electromagneticenergy source for emitting pulses of electromagnetic energy has a sondeassembly and an energy storage capacitor. The energy storage capacitorincludes an electrode mounted in the sonde assembly that is operable togenerate an electric field and a capacitive charge storage mediumsurrounding the electrode. A communication cable extends through atubular member to the electrode and the tubular member acts as a ground.A fast-closing switch is positioned between the electrode and theground.

In alternate embodiments, the sonde assembly can extend from a downholeend of the tubular member. The communication cable can extend throughthe tubular member to a surface and the communication cable can providea signal to the electrode.

In yet another alternate embodiment of this disclosure, a method foremitting pulses of electromagnetic energy with an electromagnetic energysource includes providing the electromagnetic energy source having asonde assembly, and an energy storage capacitor including an electrodemounted in the sonde assembly that is operable to generate an electricfield, and a capacitive charge storage medium surrounding the electrode.A communication cable extends through a tubular member to the electrode.A fast-closing switch is positioned such that when the fast-closingswitch is in a closed position, a circuit is formed that discharges theelectrode. The energy storage capacitor is charged to cause thefast-closing switch to close and pulses of electromagnetic energy to beemitted from the electromagnetic energy source.

In alternate embodiments the method can further include providing a highvoltage power to the electrode with a high voltage power supply that isin communication with the communication cable. The sonde assembly caninclude a downhole section axially aligned with, and spaced from, anuphole section, each of the downhole section and the uphole sectionhaving an energy storage capacitor. The communication cable can extendthrough the uphole section of the sonde assembly. Alternately, thetubular member can act as a ground. The communication cable can be incommunication with a high voltage power supply and transmits a controlsignal.

In other alternate embodiments, the electromagnetic energy source canfurther include a plurality of electromagnetic energy sources and themethod can further include emitting pulses of electromagnetic energy totravel through a subsurface hydrocarbon reservoir. The method canfurther include moving the electromagnetic energy source to a successionof locations in a well borehole for emitting the pulses ofelectromagnetic energy at the succession of locations for travel througha subsurface hydrocarbon reservoir. The capacitive charge storage mediumcan be selected that has a magnetic permeability and electricpermittivity that results in an impedance of the pulses that correspondsto an impedance of a subsurface hydrocarbon reservoir.

In another alternate embodiment of this disclosure, a method forelectromagnetic imaging of a subsurface hydrocarbon reservoir includeslowering an electromagnetic energy source into a well borehole to adepth of interest in the subsurface hydrocarbon reservoir. Theelectromagnetic energy source includes a sonde assembly, an energystorage capacitor including an electrode mounted in the sonde assemblythat is operable to generate an electric field, and a capacitive chargestorage medium surrounding the electrode. A communication cable extendsthrough a tubular member to the electrode. A fast-closing switch ispositioned such that when the fast-closing switch is in a closedposition, a circuit is formed that discharges the electrode. Pulses ofelectromagnetic energy are emitted with the electromagnetic energysource to travel through the subsurface hydrocarbon reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the recited features, aspects and advantagesof the disclosure, as well as others that will become apparent, areattained and can be understood in detail, a more particular descriptionof the embodiments of the disclosure previously briefly summarized maybe had by reference to the embodiments that are illustrated in thedrawings that form a part of this specification. It is to be noted,however, that the appended drawings illustrate only certain embodimentsof the disclosure and are, therefore, not to be considered limiting ofthe disclosure's scope, for the disclosure may admit to other equallyeffective embodiments.

FIG. 1 is a schematic section view of a transmitter-receiver array for aborehole to borehole electromagnetic survey, in accordance with anembodiment of this disclosure.

FIG. 2 is a schematic section view of an electromagnetic energy sourceand storage capacitor, in accordance with an embodiment of thisdisclosure.

FIG. 3 is a schematic cross section view of the electromagnetic energysource of FIG. 2.

FIG. 4 is a schematic cross section view of the electromagnetic energysource of FIG. 2.

FIG. 5 is a schematic section view of an electromagnetic energy sourceand storage capacitor, in accordance with an embodiment of thisdisclosure.

FIG. 6 is a schematic cross section view of the electromagnetic energysource of FIG. 5.

FIG. 7 is a schematic cross section view of the electromagnetic energysource of FIG. 5.

DETAILED DESCRIPTION OF THE DISCLOSURE

The disclosure refers to particular features, including process ormethod steps. Those of skill in the art understand that the disclosureis not limited to or by the description of embodiments given in thespecification. The subject matter of this disclosure is not restrictedexcept only in the spirit of the specification and appended Claims.

Those of skill in the art also understand that the terminology used fordescribing particular embodiments does not limit the scope or breadth ofthe embodiments of the disclosure. In interpreting the specification andappended Claims, all terms should be interpreted in the broadestpossible manner consistent with the context of each term. All technicaland scientific terms used in the specification and appended Claims havethe same meaning as commonly understood by one of ordinary skill in theart to which this disclosure belongs unless defined otherwise.

As used in the Specification and appended Claims, the singular forms“a”, “an”, and “the” include plural references unless the contextclearly indicates otherwise.

As used, the words “comprise,” “has,” “includes”, and all othergrammatical variations are each intended to have an open, non-limitingmeaning that does not exclude additional elements, components or steps.Embodiments of the present disclosure may suitably “comprise”, “consist”or “consist essentially of” the limiting features disclosed, and may bepracticed in the absence of a limiting feature not disclosed. Forexample, it can be recognized by those skilled in the art that certainsteps can be combined into a single step.

Where a range of values is provided in the Specification or in theappended Claims, it is understood that the interval encompasses eachintervening value between the upper limit and the lower limit as well asthe upper limit and the lower limit. The disclosure encompasses andbounds smaller ranges of the interval subject to any specific exclusionprovided.

Where reference is made in the specification and appended Claims to amethod comprising two or more defined steps, the defined steps can becarried out in any order or simultaneously except where the contextexcludes that possibility.

Looking at FIG. 1, an example arrangement of a transmitter-receiverarray for a borehole to borehole electromagnetic survey is shown. Thetransmitter can be electromagnetic energy source 10. Electromagneticenergy source 10 can be located within well borehole 12. Well borehole12 can extend through subsurface hydrocarbon reservoir 14.Electromagnetic energy source 10 can emit pulses of electromagneticenergy to travel through subsurface hydrocarbon reservoir 14 forelectromagnetic imaging of subsurface hydrocarbon reservoir 14.

Although one electromagnetic energy source 10 is shown in the example ofFIG. 1, in alternate embodiments, multiple electromagnetic energysources 10 can be located within well borehole 12. Alternately, one ormore electromagnetic energy sources 10 can be located at the earthsurface 15 over the subsurface hydrocarbon reservoir. In the example ofFIG. 1, a series of electromagnetic sensors 16 are located in sensorbore 18. Sensor bore 18 can be a borehole that extends throughsubsurface hydrocarbon reservoir 14, and spaced apart from well borehole12. In alternate embodiments, electromagnetic sensors 16 can be in anarray (not shown) over the earth surface 15 over subsurface hydrocarbonreservoir 14. When electromagnetic energy source 10 is located in wellborehole 12 and electromagnetic sensors 16 are located over the earthsurface 15, the arrangement is known as a borehole to surface array.Generally either or both of the electromagnetic energy source 10 andelectromagnetic sensors 16 are located within a borehole so that the EMsignals pass through subsurface hydrocarbon reservoir 14 when travelingfrom electromagnetic energy source 10 to electromagnetic sensors 16.Electromagnetic sensors 16 can form a measure of the arrival time of theemitted pulses from electromagnetic energy source 10 to image subsurfacehydrocarbon reservoir 14.

As can be seen in FIG. 1, a multitude of EM energy measurements can beperformed with different combinations of transmitter locations 20 andreceiver locations 22 in order to sample various parts of thesubterranean features from different directions, including subsurfacehydrocarbon reservoir 14. Both the electromagnetic energy source 10 andelectromagnetic sensors 16 can be a part of a downhole tool or locatedin a tool and can be movable to between a succession of locations, suchas between transmitter locations 20 and receiver locations 22.

Electromagnetic energy source 10 can be attached to installation string24 for travel in well borehole 12 to a depth of interest. Installationstring 24 extends from vehicle 26 at the surface and can be, forexample, a wireline or coiled tubing. System control unit 28 can beassociated with vehicle 26 and can be used to control the pulses emittedby electromagnetic energy source 10. A second vehicle 30 can have areceiver wireline 32 for attaching to electromagnetic sensors 16 and formoving electromagnetic sensors 16 within sensor bore 18. Electromagneticenergy source 10 can include a half wave or quarter wave dipole antenna,or a monopole antenna.

Looking at FIGS. 2-3, example embodiment of electromagnetic energysource 10 includes sonde assembly 34. Sonde assembly 34 of suchembodiment has two main sections: downhole section 34 a is axiallyaligned with, and spaced from, uphole section 34 b. Electromagneticenergy source 10 also includes energy storage capacitor 40 withcapacitive charge storage medium 44.

An electrode 42 is mounted in each of downhole section 34 a and upholesection 34 b of sonde assembly 34. Downhole electrode 42 a is located indownhole section 34 a and uphole electrode 42 b is located in upholesection 34 b. Electrode 42 can be an elongated member and have a tubularshape. Electrode 42 can be formed of copper, and in alternateembodiments, can be formed of silver, aluminum, gold, graphite, or othermaterial with sufficient conductivity, corrosion resistance and hardnesssuitable for use as an electrode. Downhole electrode 42 a can be part ofsonde assembly 34. Downhole electrode 42 a can include a downhole opencentral bore 43 a. In alternate embodiments, downhole electrode 42 a canbe a solid tubular shaped member. Uphole electrode 42 b can be part ofsonde assembly 34. Uphole electrode 42 b can include uphole open centralbore 43 b so that uphole electrode 42 b is a tubular member. Uphole opencentral bore 43 b can include an opening proximate to an uphole end ofuphole electrode 42 b so that communication cable 45 can pass intouphole open central bore 43 b of uphole electrode 42 b.

Communication cable 45 can travel through uphole open central bore 43 band exit out of uphole open central bore 43 b proximate to a downholeend of uphole electrode 42 b. Communication cable 45 can be incommunication with high voltage power supply 48 and can transmit acontrol signal from system control unit 28 (FIG. 1) to control thepulses emitted by electromagnetic energy source 10.

Each half of the dipole is initially held at a high voltage relative toone another. The voltage will be dependent on the capacitance andimpedance of the circuit, as well as dimensions of the antenna, and willchange with the frequency of operation of the antenna. The voltage ofone half of the dipole is equal and opposite of the voltage of the otherhalf of the dipole, with the outermost end of each half therefore havingthe greatest magnitude of voltage. As used in this disclosure, “highvoltage” can a voltage in a range 1000 volts (V) and greater. The pairof antennas is biased apart by a large voltage so that the structure candischarge in a single massive current pulse and emit a high powertransient signal.

The capacitive charge storage medium 44 is mounted in each of thedownhole section 34 a and the uphole section 34 b of the sonde assembly34. Capacitive charge storage medium 44 can be formed of a material thatis selected so that the electric permittivity and magnetic permeabilityof such material optimizes transmitter impedance to match the externalmedium. This increases the capacitance and inductance of the system anddecreases the group velocity of the pulses emitted by electromagneticenergy source 10, to define a slow-wave antenna.

A slow wave refers to the group velocity of the EM wave travelling alongthe structure. The group velocity can be made slower by changing theelectromagnetic properties of the guiding structure or in particular thecladding around the guidewire according to the formula:

$V \propto \frac{1}{\sqrt{ɛ_{r}\mu_{r}}}$where V=wave velocity, εr is the dielectric constant, and μr is therelative magnetic permeability.

The slower group velocity of the EM waves allows a proportionalreduction in the physical length of the antenna. A slower group velocityis especially important for lower frequency applications, such asapplications with frequencies in the 10 kHz-1 MHz range, with very largewavelengths, such as where the length of the antenna corresponding toone quarter of the resonant wavelength of the non-cladded antenna ishundreds of meters.

Providing a capacitive charge storage medium 44 of embodiments of thisdisclosure can materially decrease the length of an antenna structurefor a given wavelength emitted. As an example, capacitive charge storagemedium 44 can be formed of a material that includes ferrite, steel,permalloy, TiO2, lead zirconate titanate, magnetite, other ironparticles, or a mix of any such materials. Such particles 44 a can bemixed in an epoxy matrix 44 b. The specific composition of the mixtureused for capacitive charge storage medium 44 would depend on theproperties of the reservoir materials and the geometry of the antenna,as further described in this disclosure. In an example embodiment,capacitive charge storage medium 44 can have particles 44 a with arelative electric permittivity of 100 and a relative magneticpermeability of 100 and consist of both TiO2 and magnetite. The relativeelectric permittivity of a material is known as the dielectric constantof the material divided by the dielectric constant of free space. Inother alternate embodiments, the relative magnetic permeability ofparticles 44 a can be in a range of 1 to 100,000. These particles 44 acan be located in a 1:1 mixture in an insulating epoxy matrix 44 b. Thisexample embodiment would result in an overall relative electricpermittivity in the range of 40, and a magnetic permeability in therange of 40 after considering the linear combination of the components,in accordance with effective medium theory. This example embodiment willresult in an effective antenna that performs as though it is in therange of forty times larger than the actual length of the antenna.

Looking at FIGS. 2-4, electromagnetic energy source 10 can furtherinclude fast-closing switch 46, which is located between downhole anduphole electrodes 42 a, 42 b of downhole and uphole sections 34 a, 34 b,respectively. When fast-closing switch is closed, such as when the sparkgap is broken down, electromagnetic energy source 10 will generate anelectromagnetic pulse. Fast-closing switch 46 is positioned such thatwhen fast-closing switch 46 is in a closed position, a circuit istransiently formed that connects electrodes 42 a and 42 b. Fast-closingswitch 46 can be, for example, a spark gap.

As an example, when the potential difference between downhole and upholeelectrodes 42 a, 42 b exceeds the breakdown voltage of a gas within thegap, an electric spark can pass between downhole and uphole electrodes42 a, 42 b. In alternate embodiments, fast-closing switch 46 can includeavalanche transistors, thyratrons, ignitrons, silicon-controlledrectifier, and triggered spark gaps. Fast-closing switch 46 can beselected to have performance metrics concerning peak current, peakvoltage, useful number of shots, jitter, complexity and geometry thatwill suit the environment, conditions, and performance criteria forwhich the electromagnetic energy source 10 is to be used.

Electromagnetic energy source 10 can also have high voltage power supply48 connected between downhole and uphole electrodes 42 a, 42 b. Highvoltage power supply 48 can have, for example, a voltage over 1,500volts. Power can be provided to high voltage power supply 48 fromoutside of electromagnetic energy source 10 with pair of leads and byway of communication cable 45.

In the example embodiment of FIGS. 2-4, capacitive charge storage medium44 acts as a ground. In such an embodiment, capacitive charge storagemedium 44 proximate to electrode 42 will form energy storage capacitor40 and capacitive charge storage medium 44 proximate to an outerdiameter of capacitive charge storage medium 44 will act as the ground.

Current limiting resistors 50 can be located between high voltage powersupply 48 and both of the downhole electrode 42 a of the downholesection 34 a and the uphole electrode 42 b of the uphole section 34 b.Current limiting resistors 50 can block current pulses from returning upthe supply wire towards high voltage power supply 48. This will isolatethe antenna system from high voltage power supply 48 while theelectromagnetic pulse is being emitted.

Electrode 42 is centered along axis Ax of each of downhole section 34 aand uphole section 34 b of sonde assembly 34. Electrode 42 is sheathedwithin capacitive charge storage medium 44 so that capacitive chargestorage medium 44 surrounds electrode 42. Energy storage capacitor 40 isformed by an electric field radiating out from electrode 42 and throughthe nearby capacitive charge storage medium 44. The amount of energystored will vary with the square of the electric field. If electrode 42has a small diameter, then almost all of the electric field potentialdrop will occur inside the capacitive charge storage medium 44.

Looking at FIGS. 5-7, electromagnetic energy source 10 can alternatelyhave a single electrode 42 of sonde assembly 34. In such an embodiment,communication cable 45 can extend downhole through a tubular member suchas casing or coiled tubing 52 to high voltage power supply 48. Highvoltage power supply 48 is in communication with electrode 42. Electrode42 is sheathed within capacitive charge storage medium 44 so thatcapacitive charge storage medium 44 surrounds electrode 42. Energystorage capacitor 40 is formed by an electric field radiating out fromelectrode 42 and through the nearby capacitive charge storage medium 44,that can have particles 44 a that are mixed in epoxy matrix 44 b.

Fast-closing switch 46, which is located between high voltage powersupply 48 and coiled tubing 52, which acts as a ground forelectromagnetic energy source 10. Fast-closing switch 46 is positionedsuch that when fast-closing switch 46 is in a closed position, a circuitis formed that discharges electrode 42. When fast-closing switch isclosed, such as when the spark gap is broken down, electromagneticenergy source 10 will generate an electromagnetic pulse. Such aconfiguration can be particularly useful for permanent deployment invertical wells where the well has casing to the top of the productivezone, while leaving subsurface hydrocarbon reservoir 14 open-hole. Sucha configuration is may also be particularly useful for long termreservoir monitoring and estimating an average remaining oil columnbetween adjacent wells.

Electromagnetic energy source 10 of embodiments of FIGS. 5-7 operates asa grounded half dipole antenna, or a monopole antenna. Because, thelength of the antenna affects the resonant frequency, using the singleelectrode system of FIGS. 5-7 will affect the radiation efficiency as afunction of wavelength. In addition, because the power density of theresulting pulse is a function of the direction of travel of the pulse,the use of the embodiment of FIGS. 5-7 will result in a radiationpattern that is different than the radiation pattern of the dipolearrangement of FIGS. 2-4. As an example, the vertical radiation patternof the grounded dipole antenna, or monopole antenna, has a smaller angleof radiation giving it a longer-range propagation advantages atfrequencies less than about 50 MHz as compared to the half wave dipoleantenna of FIGS. 2-4.

In EM transmission systems, a factor to be considered is matching theimpedance of all components in the system to maximize transferefficiency and minimize reflection and ringing between each element. Byincluding high voltage power supply 48 and fast-closing switch 46 assubterranean component of electromagnetic energy source 10, atransmission line that transmits the pulse from the surface iseliminated, which in turn eliminates a cause of ringing and inefficiencyin the system.

An additional factor to be considered for improving the performance ofEM transmission systems is matching the impedance of electromagneticenergy source 10 to the medium through which the pulses are transmitted.Subsurface geological structures can be comprised of rocks with porosityranging from zero to about 25 percent (%). The pore space can be filledwith water, which may have dissolved salts that affect the conductivityof the geological structure. Other pore spaces can be filled with liquidor gas hydrocarbons. Taken together, the rock matrix in and aroundsubsurface hydrocarbon reservoir 14 can have a dielectric constantranging from about 5 (such as for anhydrite or oil filled sandstone), toabout 22 (such as for water filled limestone with 20% porosity).

The impedance (Z) of a medium can be calculated as shown in thefollowing Equation 1:

$\begin{matrix}{Z = \sqrt{\frac{\mu}{ɛ}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where μ is the magnetic permeability and c is the electric permittivityof the medium. As shown in Equation 1, the impedance of a medium isdetermined by the ratio of magnetic permeability to electricpermittivity (μ/ε ratio).

The dielectric constant (k) of a material can be calculated as shown inEquation 2:

$\begin{matrix}{k = {{ɛ\; r} = \frac{ɛ}{ɛ_{o}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$so then,ε=εrεo=kεo  Equation 3where εo is the electric permittivity of free space. Using Equation 3with Equation 1, the relationship between impedance and dielectricconstant can be expressed as shown in Equation 4:

$\begin{matrix}{Z = \sqrt{\frac{\mu}{k\; ɛ_{o}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$In Equation 4 it is shown that as the dielectric constant k increases,impedance Z decreases. And as magnetic permeability μ increases, so doesthe impedance Z of the medium.

Some current EM transmission systems are designed to match the impedanceof free space, which is about 377Ω. Given their higher dielectricconstant compared to free space, the impedance of the oil and brinefilled portions of the reservoir can be in a range from about 80-170ohms (Ω), depending on whether the reservoir includes brine or oilfilled pores. Embodiments of the current application improve theperformance of electromagnetic energy source 10 compared to EMtransmission systems with an impedance of about 377Ω by tuning the ratioof magnetic permeability to electric permittivity while maintaining adesired product of magnetic permeability multiplied by electricpermittivity, for miniaturization.

The materials used for capacitive charge storage medium 44 forelectromagnetic energy source 10 can be selected depending on whetherelectromagnetic energy source 10 is to be used for oil or water filledrocks. As an example, if electromagnetic energy source 10 is to be usedto survey water-flooded reservoirs, μ and ε for electromagnetic energysource 10 would be selected to have a μ/ε ratio in a range of 6400 to100,000. Alternately, if electromagnetic energy source 10 is to be usedto survey hydrocarbon producing reservoirs, μ and ε for electromagneticenergy source 10 would be selected to have a μ/ε ratio of 23000 to30000.

Considering instead the ratio of relative magnetic permeability torelative electric permittivity, the relative magnetic permeability ofthe reservoir could be about 1. Depending on the porosity and amount ofresidual water, the relative electric permittivity of the reservoircould range from 10 for an original hydrocarbon filled reservoir, to 20for a water filled or swept reservoir. Therefore, to optimizetransmission efficiency, desirable ratios of relative magneticpermeability to relative electric permittivity in the sheathing materialin the EMU antenna should be close to the ratio of the reservoir medium,such as in the range of 0.05 to 0.1.

The ratio of magnetic permeability to electric permittivity can beselected by using a mixture of magnetic and dielectric materials as partof capacitive charge storage medium 44, such as titania and magnetite aswell as special ferrites that simultaneously display both increasedmagnetic permeability and electric permittivity. The ratio of magneticpermeability to electric permittivity can be tuned by adjusting thestoichiometry of the various metals in the ferrite. In such a way,capacitive charge storage medium 44 can have a magnetic permeability andelectric permittivity selected to result in an impedance ofelectromagnetic energy source 10 that corresponds to an impedance ofsubsurface hydrocarbon reservoir 14.

As an example, a material with tunable properties can be used as part ofcapacitive charge storage medium 44. Such a material can be a ferritethat has the composition Ti_(x)(MFe₂O_(4+2x/y))_(y), where x+y=1 and0<x<1. The metal M can be any one or several metals selected from thegroup of Mn, Ni, Cu, Mg, Li and Zn. Considering a ferrite withM=Ni(12%)+Cu(28%)+Zn(60%) as an example, such a composition is thenTi_(x)(Ni_(0.12)Cu_(0.28)Zn_(0.6)Fe₂O_(4+2x/y))_(y). In embodiments ofthis application, such a ferrite with an x=0.05 fraction of Ti can beused for capacitive charge storage medium 44. Such a composition forcapacitive charge storage medium 44 can maximize the product of magneticpermeability multiplied by electric permittivity, while maintaining theratio of relative magnetic permeability to relative electricpermittivity of about 0.10.

In some current EM transmission systems, the cable used to transmitpower and control signals downhole to electromagnetic energy source 10can be routed exterior of at least one of the electrodes 42. The use ofsuch an external cable will introduce asymmetry into the radiationpattern from the electromagnetic energy source 10, which couldnegatively impact data quality and the interpretation of such data.Embodiments of this disclosure eliminate such asymmetry by eitherpassing communication cable 45 through a tubular member to electrode 42.The tubular member through which communication cable 45 passes can be acentral bore of uphole electrode 42 b (FIG. 2), or can be casing orcoiled tubing 52 in an embodiment having only one electrode 42, which islocated downhole of communication cable 45 (FIG. 5).

In an example of operation, in order to form an electromagnetic image ofsubsurface hydrocarbon reservoir 14 electromagnetic energy source 10 canbe mounted to, or part of, a well tool and lowered on installationstring 24 into well borehole 12 to a depth of interest. Capacitivecharge storage medium 44 of electromagnetic energy source 10 can beselected to have a magnetic permeability and electric permittivityselected to result in an impedance of electromagnetic energy source 10that corresponds to an impedance of the subsurface hydrocarbonreservoir.

The downhole tool associated with electromagnetic energy source 10 canhave a connector section with a mechanical connector that attaches toinstallation string 24, an electrical power connection, and asynchronizing signal connection. Such connector section and connectionscan be orientated similar to known current downhole tools. The downholetool can house sonde assembly 34. Electromagnetic energy source 10 canbe encased in a strong, insulating polymeric material to providestructural integrity while also allowing for the transmission ofelectromagnetic signals.

A single electromagnetic energy source 10 can be utilized, as shown inthe example of FIG. 1. Alternately, a plurality of electromagneticenergy sources 10 can be lowered in well borehole 12. Pulses ofelectromagnetic energy can be emitted from the single electromagneticenergy source 10, or at each of the plurality of electromagnetic energysources 10, as applicable, to travel through subsurface hydrocarbonreservoir 14 and a resulting signal can be received by electromagneticsensors 16. An electromagnetic pulse with known characteristics isgenerated from the high power, pulsed electromagnetic energy source 10from locations in or near subsurface hydrocarbon reservoir 14. In orderto generate the electromagnetic pulse, high voltage power supply 48charges up energy storage capacitor 40 through current limiting resistor50 until fast-closing switch 46 is closed. With the fast-closing switchclosed, electromagnetic energy source 10 will emit the pulse ofelectromagnetic energy. After the electromagnetic pulse is emitted, highvoltage power supply 48 can recharge energy storage capacitor 40.

By combining energy storage, pulse formation and radiating elements intoa single structure, the problem of impedance matching between separatedistributed components of an electromagnetic survey system required forthese respective functions is eliminated. Systems and methods of thisdisclosure therefore eliminate the problem of load matching between apower supply, cable or transmission-line, and antenna. With the energystorage element of energy storage capacitor 40 and fast-closing switch46 both inside the transmitting antenna element of the pair of disclosedself-powered impulse antennas, the need for an external cable betweenthe power source and the transmission element are eliminated, andreflections and losses in the system are minimized.

In addition, because the cable that provides a control signal toelectromagnetic energy sources 10 and is associated with high voltagepower supply 48 does not extend past an electrode 42, the resultingradiation pattern from the electromagnetic energy source 10 will haveimproved symmetry compared to systems where a power or communicationcable extends exterior of an electrode 42.

A plurality of electromagnetic sensors 16 can be mounted to or part of awell tool and lowered in sensor bore 18 that extends through subsurfacehydrocarbon reservoir 14. Alternately, the plurality of electromagneticsensors 16 can be arranged in an array over an earth surface 15 oversubsurface hydrocarbon reservoir 14. The emitted pulsed EM signal istransmitted through subsurface hydrocarbon reservoir 14 and recorded atone or more electromagnetic sensors 16 after travel through thesubsurface formations surrounding well borehole 12 and sensor bore 18.The EM signal recorded by electromagnetic sensors 16 differs from thepulsed signal emitted by electromagnetic energy source 10 incharacteristics such as time, amplitude, power spectrum, and othercharacteristics that depend on the properties of the intervening medium(such as the reservoir) and spatial variations of those properties.

Electromagnetic energy source 10 can be moved between a succession oflocations, such as transmitter locations 20, in well borehole 12 foremitting pulses of electromagnetic energy at such succession oflocations for travel through subsurface hydrocarbon reservoir 14.Similarly, electromagnetic sensors 16 can be moved between a successionof locations, such as receiver locations 22, to receive the resultingsignal at such succession of locations. In this way, a more completeelectromagnetic image can be formed of subsurface hydrocarbon reservoir14.

Recording and processing instrumentation associated with system controlunit 28 at the surface can receive and store information relating to theresulting signal received by electromagnetic sensors 16. System controlunit 28 can also perform additional functions such as computerizedanalysis of the resulting signal, display certain results derived fromthe resulting signal, and store the resulting signal and computerizedanalysis on a computer for further processing and computerized analysis.System control unit 28 can, as an example, be used to form a measure ofthe arrival time of the emitted pulses at a plurality of electromagneticsensors, and to analyze the measure of arrival time data from theplurality of electromagnetic sensors. From this information, arepresentation of subsurface features of the subsurface hydrocarbonreservoir, and an image of the representation of subsurface features ofthe subsurface hydrocarbon reservoir, can be formed.

Embodiments of this disclosure thus generate information about thespatial distribution and composition of fluids in a hydrocarbonreservoir. The operation can be repeated periodically to, as an exampledetermine the direction, velocity and saturation of injected fluids,such as a water flood, or to visualize modified reservoir volume as afunction of time. This can assist in optimizing reservoir management,preventing oil bypass and thereby improving volumetric sweep efficiencyand production rates.

Embodiments of this disclosure have been sufficiently described so thata person with ordinary skill in the art may reproduce and obtain theresults mentioned in this disclosure. Nonetheless, any skilled person inthe field of technique, subject of this disclosure, may carry outmodifications not described in this disclosure, to apply thesemodifications to a determined structure, or in the manufacturing processof the same, and such resulting structures shall be covered within thescope of this disclosure.

It should be noted and understood that there can be improvements andmodifications made of the present embodiments described in detail inthis disclosure without departing from the scope of the disclosure.

What is claimed is:
 1. A method for emitting pulses of electromagneticenergy with an electromagnetic energy source, the method comprising:providing the electromagnetic energy source having: a sonde assembly; anenergy storage capacitor including an electrode mounted in the sondeassembly and operable to generate an electric field, and a capacitivecharge storage medium surrounding the electrode; a communication cableextending through a tubular member to the electrode; and a fast-closingswitch positioned such that when the fast-closing switch is in a closedposition, a circuit is formed that discharges the electrode; andcharging the energy storage capacitor to cause the fast-closing switchto close and pulses of electromagnetic energy to be emitted from theelectromagnetic energy source.
 2. The method of claim 1, furtherincluding providing a high voltage power to the electrode with a highvoltage power supply that is in communication with the communicationcable.
 3. The method of claim 1, where the sonde assembly includes adownhole section axially aligned with, and spaced from, an upholesection, each of the downhole section and the uphole section having anenergy storage capacitor.
 4. The method of claim 3, where thecommunication cable extends through the uphole section of the sondeassembly.
 5. The method of claim 1, where the tubular member acts as aground.
 6. The method of claim 1, where the communication cable is incommunication with a high voltage power supply and transmits a controlsignal.
 7. The method of claim 1, where the electromagnetic energysource further includes a plurality of electromagnetic energy sourcesand the method further includes emitting pulses of electromagneticenergy to travel through a subsurface hydrocarbon reservoir.
 8. Themethod of claim 1, further including moving the electromagnetic energysource to a succession of locations in a well borehole for emitting thepulses of electromagnetic energy at the succession of locations fortravel through a subsurface hydrocarbon reservoir.
 9. The method ofclaim 1, further including selecting the capacitive charge storagemedium that has a magnetic permeability and electric permittivity thatresults in an impedance of the pulses that corresponds to an impedanceof a subsurface hydrocarbon reservoir.
 10. A method for electromagneticimaging of a subsurface hydrocarbon reservoir, the method comprising:lowering an electromagnetic energy source into a well borehole to adepth of interest in the subsurface hydrocarbon reservoir, theelectromagnetic energy source including: a sonde assembly; an energystorage capacitor including an electrode mounted in the sonde assemblyand operable to generate an electric field, and a capacitive chargestorage medium surrounding the electrode; a communication cableextending through a tubular member to the electrode; and a fast-closingswitch positioned such that when the fast-closing switch is in a closedposition, a circuit is formed that discharges the electrode; andemitting pulses of electromagnetic energy with the electromagneticenergy source to travel through the subsurface hydrocarbon reservoir.